The present invention relates to a glow plug for preheating a subcombustion or combustion chamber of a diesel engine and, more particularly, to an improvement of a diesel engine glow plug having a ceramic heater which has fast heating and self temperature saturation properties and which allows "after glow" operation for a long period of time.
Conventional glow plugs having various types of structures have been proposed. Among these glow plugs, a plug having a ceramic heater has received a great deal of attention as a fast heating plug.
A plug of a ceramic heater type is described in Japanese Patent Laid-Open No. 60-14784. In this glow plug, a resistive ceramic material having substantially the same thermal expansion coefficient as that of an insulating ceramic material constituting a heater insulating element is used. A heating element is exposed on the outer surface of a heater and the heating element is integrally formed with the heater insulating element. With this structure, the distal end of the heater can be immediately heated to obtain a fast heating glow plug. At the same time, bonding between the heating element and the heater insulating element can be optimally and appropriately maintained to improve reliability for heat resistance or the like to some extent.
In a conventional glow plug of a ceramic heater type having the structure as described above, however, many problems are left unsolved from the structural and functional points of view when such a plug is used as a practical glow plug.
With the above structure, the heating element is exposed on the surface of the heater. Although this heater serves as a fast heating heater to some extent, the heating element has a U-shaped laminated construction and both ends are guided to the rear end portion of the heater. In order to prepare a practical heater, a special care must be paid to the structures of electrode extraction portions and a holding portion for a holder. For example, the simplest method of extracting electrodes from a U-shaped ceramic heater is to externally connect one of the lead portions through an external connecting terminal kept insulated from the rear end portion of the holder and to ground the other lead portion through a conductive layer such as a metallized layer formed on the holding portion fitted in the holder. When the above structure is employed, an insulating layer such as an insulating coating layer must be formed on the holding portion of one lead portion which is to be fitted in the holder, so that the holding portion is kept insulated. In this case, the thickness of the insulating layer must be 20 .mu.m or more. This thickness is larger than the thickness of the conductive layer (i.e., the thickness of the conductive layer is 5 .mu.m or less). It is difficult or time-consuming to form these layers on the corresponding lead portion. In addition, since the layers having different thicknesses are formed, it is difficult to coaxially fit the lead portion in the holder, thus causing assembly problems.
The portion to be connected to the holder through such conductive and insulating layers must have high heat resistance and good hermetic seal free from a thermal influence as well as high electrical conductivity and insulating properties. In this manner, many quality check items are required for the lead holding portion. It is difficult to obtain lead holding portions having uniform quality. Nonuniform quality poses many practical problems. Demand has arisen for developing a glow plug having a ceramic heater, fast heating and self temperature saturation properties, and high reliability such as high heat resistance.